Thermally tuned coaxial cable for microwave antennas

ABSTRACT

A coaxial cable, including an outer conductor and an inner conductor adapted to connect to an energy source for treating tissue and first and second dielectric materials disposed between the inner conductor and the outer conductor which position the inner conductor relative to the outer conductor in general concentric relation thereto. The first dielectric material has a first coefficient of thermal expansion and the second dielectric material has a second coefficient of thermal expansion different from the first coefficient of thermal expansion.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority to U.S. Provisional Patent Application Ser. No. 61/023,029, titled “THERMALLY TUNED COAXIAL CABLE FOR MICROWAVE ANTENNAS” filed Jan. 23, 2008 by Kenlyn Bonn, which is incorporated by reference herein.

BACKGROUND

1. Technical Field

The present disclosure relates generally to microwave antennas. More particularly, the present disclosure relates to thermally tuning coaxial cables for microwave antennas.

2. Background of Related Art

Microwave antennas are used in many applications. For example, medical microwave ablation antennas are used by surgeons. In fact, ablation devices utilizing DC shock, radio frequency (RE) current, ultrasound, microwave, direct heat, or lasers have been introduced and employed to various degrees to ablate biological tissues. Ablation devices may be used in open surgical procedures or are sometimes inserted into catheter devices in order to perform laparoscopic ablation procedures. The catheter incorporating the ablation device is generally inserted into a major vein or artery or through a body cavity. These catheters are then guided to a targeted location in the body (e.g., organ) by manipulating the catheter from the insertion point or the natural body orifice.

During ablation, the dielectric constant of the tissue changes as more water is boiled off and tissue desiccation occurs. The changing value of the dielectric constant alters the antenna's ability to match the originally designed impedance of the antenna. In addition, during microwave ablation in tissue, the impedance of the tissue varies during the course of ablation. This occurrence directly corresponds to how much energy has been deposited into the tissue during the ablation, resulting in temperature increases at the ablation site.

The impedance in the coaxial cable is typically related to the concentricity of the inner conductor in relationship to the outer conductor. In ablation procedures, however, conventional antenna designs only allow for an initial impedance match and as ablation occurs, the increase in mismatch between the tuning point of the antenna and the ablated tissue reduces the efficiency of the energy deposition in the tissue.

SUMMARY

The present disclosure relates to a coaxial cable. The coaxial cable includes an outer conductor and an inner conductor adapted to connect to an energy source for treating tissue, and first and second dielectric materials disposed between the inner conductor and the outer conductor which position the inner conductor relative to the outer conductor in general concentric relation thereto. The first dielectric material has a first coefficient of thermal expansion and the second dielectric material has a second coefficient of thermal expansion different from the first coefficient of thermal expansion.

In another embodiment, a coaxial cable also includes one or more dielectric spacer(s) disposed between the inner conductor and the outer conductor. The dielectric spacer(s) include first and second dielectric materials disposed between the inner conductor and the outer conductor, which position the inner conductor relative to the outer conductor in general concentric relation thereto. The first dielectric material has a first coefficient of thermal expansion and the second dielectric material has a second coefficient of thermal expansion different from the first coefficient of thermal expansion.

In an example embodiment, a heating element is disposed within the dielectric material and is adapted to connect to an energy source for controlling the temperature thereof. The heating element heats the dielectric material to cause thermal expansion thereof.

The present disclosure also relates to a method for controlling the impedance of coaxial cable used to treat tissue, the method includes the steps of: providing an outer conductor and an inner conductor adapted to connect to an energy source for treating tissue. The first and second dielectric materials are disposed between the inner conductor and the outer conductor and position the inner conductor relative to the outer conductor in general concentric relation thereto. The first dielectric material has a first coefficient of thermal expansion and the second dielectric material has a second coefficient of thermal expansion different from the first coefficient of thermal expansion. The method also includes the steps of energizing the cable and determining the impedance of the inner conductor, and regulating the change of impedance of the inner conductor by selectively heating at least one of the first and second dielectric materials causing thermal expansion thereof to move the inner conductor relative to the outer conductor to change the impedance of the coaxial cable.

Thus, by using dielectric cores of varying thermal expansion values, it is possible to force the eccentricity of the inner conductor of the coaxial cable on-line or off-line effectively changing the coaxial cable's impedance value.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the present disclosure are described herein with reference to the drawings wherein:

FIG. 1A is a front, perspective view of a centrally-disposed coaxial cable having an inner conductor held by two materials having different coefficient of thermal expansion values, in accordance with an embodiment of the present disclosure;

FIG. 1B is a front, perspective view of an off-center coaxial cable having an inner conductor held by two materials having different coefficient of thermal expansion values, in accordance with another embodiment of the present disclosure;

FIG. 1C is a schematically-illustrated, cross-sectional view of the coaxial cable of FIG. 1A;

FIG. 1D is a schematically-illustrated, cross-sectional view of the coaxial cable of FIG. 1B;

FIG. 2A is front, perspective view of an off-centered coaxial cable having an inner conductor held by two materials having different coefficient of thermal expansion values, in accordance with another embodiment of the present disclosure;

FIG. 2B is a front, perspective view of a centrally disposed coaxial cable having an inner conductor held by two materials having different coefficient of thermal expansion values, in accordance with another embodiment of the present disclosure;

FIG. 2C is a schematically-illustrated, cross-sectional view of the coaxial cable of FIG. 2B;

FIG. 2D is a schematically-illustrated, cross-sectional view of the coaxial cable of FIG. 2A;

FIG. 3 is a schematically illustrated, cross-sectional view of a coaxial cable having an inner conductor held by one or more spacers being composed of one or more materials having different coefficient of thermal expansion values, in accordance with another embodiment of the present disclosure;

FIG. 4A is a front, perspective view of an off-centered coaxial cable having an inner conductor and a plurality of resistive heating elements in each of two or more materials having different coefficient of thermal expansion values, in accordance with another embodiment of the present disclosure;

FIG. 4B is a front, perspective view of a centrally disposed coaxial cable having an inner conductor and a plurality of resistive heating elements in each of two or more materials having different coefficient of thermal expansion values, in accordance with another embodiment of the present disclosure;

FIG. 5A is a schematically illustrated cross-sectional view of an off-centered coaxial cable having an inner conductor and a plurality of resistive heating elements in one material having one coefficient of thermal expansion value, in accordance with another embodiment of the present disclosure;

FIG. 5B is a front, perspective view of a centrally disposed coaxial cable having an inner conductor and a plurality of resistive heating elements in one material having one coefficient of thermal expansion value, in accordance with another embodiment of the present disclosure;

FIG. 6 is a front, perspective view of a coaxial cable having an inner conductor with a shape memory alloy, in accordance with another embodiment of the present disclosure; and

FIG. 7 is a front, perspective view of a centrally-disposed coaxial cable having an inner conductor held by two materials having different coefficient of thermal expansion values with a fluid circulated therethrough for regulating the thermal expansion of the two materials in accordance with another embodiment of the present disclosure.

DETAILED DESCRIPTION

To achieve the foregoing and other objects of the present disclosure, methods and devices pertaining to the microwave antennas are disclosed. In general, the present disclosure pertains to a coaxial cable assembly and, in one embodiment, to a surgical device including the coaxial cable assembly. The surgical device generally includes an ablative energy source and an ablative energy delivery device coupled to the ablative energy source. The ablative energy delivery device is configured to deliver ablative energy sufficiently strong enough to cause tissue ablation. In most embodiments, the ablative energy is formed from electromagnetic energy in the microwave frequency range. Other applications are contemplated by the present disclosure, such as telecommunications or other suitable applications in which microwave antennas are utilized.

Particular embodiments of the present disclosure are described hereinbelow with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail to avoid obscuring the present disclosure in unnecessary detail. Those skilled in the art will understand that the present disclosure may be adapted for use with either an endoscopic instrument or an open instrument.

While the present disclosure is susceptible to embodiments in many different forms, there is shown in the drawings and will be described herein in detail one or more embodiments of the present disclosure. However, the present disclosure is to be considered an exemplification of the principles of the present disclosure, and the embodiment(s) illustrated is/are not intended to limit the spirit and scope of the present disclosure and/or the claims herein.

With reference to the drawings, the coaxial cable of the particular embodiments of the present disclosure are shown. The cable may be of any suitable length, and the figures are not intended to limit the length of the cable to a specific length illustrated or any specific length. Instead, only a representative portion or section of cable is illustrated.

Referring to the embodiment of FIGS. 1A and 1B, the coaxial cable 10 includes an outer conductor 12, an inner conductor 14, a first material 16, a second material 18, a first air gap 20, and a second air gap 22. The inner conductor 14 is connected to an external power source 300.

The coaxial cable 10 may be rigid, rigid-but shapeable or flexible. The coaxial cable 10 may be chosen from commercially available standards and is generally designed with a characteristic impedance of 50 Ohms. In addition, one side of the coaxial cable 10 may be coupled to a power supply 300. Also, the other side of the coaxial cable 10 may be coupled to an antenna (not shown) in any suitable manner.

The outer conductor 12 is arranged to be generally concentric with respect to the inner conductor 14. However, the concentric relationship may be configured to meet a particular purpose as explained in more detail below. Inner conductor 14 is a central conductor used for transmitting signals and is typically held relative to the outer conductor 12 by first material 16 and second material 18. In one embodiment, the first material 16 holds the inner conductor 14, whereas the second material 18 supports the first material 16 without contacting the inner conductor 14. In other words, only one material contacts the inner conductor 14.

In the illustrated embodiment, the first material 16 and the second material 18 define first and second air gaps 20, 22 between the inner surface of the outer conductor 12 and the outer surface of the inner conductor 14. The first air gap 20 separates a first portion of the first material 16 and a first portion of the second material 18. The second air gap 22 separates a second portion of the first material 16 with a second portion of the second material 18.

The inner conductor 14 has a significant effect on the coaxial cable's 10 properties, such as the cable's 10 impedance and attenuation characteristics. The impedance on the coaxial cable 10 is related to the concentricity of the inner conductor 14 in relationship to the outer conductor 12. In the first embodiment, a thermal increase to the coaxial cable 10 is used to alter the alignment concentricity of the inner conductor 14 in a manner that would better match a change in tissue impedance. The coaxial cable 10 in the antenna (not shown) would start with an initial impedance match to a transmission line interface that would gradually taper along the length of the antenna toward a desired impedance with either the addition or the subtraction of heat. The taper could be controlled thermally through additional features, such as a cooling jacket or cooling channels.

FIGS. 1A and 1C illustrate the inner conductor 14 in a centered position within the coaxial cable 10. As heat is applied, the inner conductor 14 is moved to an off-centered position due to the thermal expansion of material 18, as shown in FIGS. 1B and 1D. As the tissue impedance changes, the alignment sensitivity of the cable 10 may be selectively changed (e.g., automatically or manually) such that the impedance of the cable 10 better matches the tissue impedance. One or more materials with different coefficients of thermal expansion may be utilized which mutually cooperate to tune the inner conductor 14 according to a desired setting, such as an ohmage setting.

FIGS. 2A and 2D show an off-centered coaxial cable 110 and FIGS. 2B and 2C show a centrally disposed coaxial cable 110 having an inner conductor held by two materials having different coefficient of thermal expansion values. The coaxial cable 110 includes an outer conductor 112, an inner conductor 114, a first material 116 and a second material 118. The inner conductor 114 is connected to an external power source 300.

The first material 116 has a first coefficient of thermal expansion value and the second material 118 has a second coefficient of thermal expansion value, the first and second coefficient of thermal expansion values being different. During heat transfer, the energy that is stored in the intermolecular bonds between atoms changes. When the stored energy increases, so does the length of the molecular bond. As a result, materials typically expand in response to heating and contract on cooling. This response to temperature change is expressed as the materials coefficient of thermal expansion. The coefficient of thermal expansion is used in two ways: (1) as a volumetric thermal expansion coefficient and (2) as a linear thermal expansion coefficient.

Therefore, when the temperature applied to the coaxial cable 110 changes, the first material 116 expands at a first rate/volume and the second material 118 expands at a second rate/volume. Typical materials used in coaxial cables include variations of PTFE, polyethylene (PE) blends and silica dioxides, however, nearly any thermo-set or thermoplastic with a low dielectric constant can be used in conjunction with another material of similar dielectric constant with a different coefficient of thermo-expansion. Typically, different polymer grades or blends result in varying material properties so determining the desired pair of materials would be a result of finding a matching mixture. The heat generated by the losses in the dielectric material in the cable can also be utilized to heat material enough to generate the differential in thermal expansion between the varying materials. A variety of different materials with different coefficient of thermal expansion values may be utilized, e.g., ABS Polymer Extruded, ABS Polymer Nylon Blend, PEEK Polyketone, PEKK Polyketone, Nylon PTFE Filled, Polycarbonate Extruded, LDPE (Polyethylene), Polyimide, PTFE Molded, Silica Aerogel and combinations thereof.

If the first material 116 expands due to a temperature increase, the second material 118 contracts due to the differing coefficient of thermal expansion values of the two materials 116, 118. As a result, as the ablation zone heats up, the difference in expansion between the two materials 116, 118 would cause the inner conductor 114 to change alignment with the outer conductor 112, e.g., move toward a centered position as illustrated in FIGS. 2B and 2C.

As can be appreciated, the materials 116, 118 may be designed to selectively (e.g., either automatically or manually) align or misalign the inner conductor 114 relative to the outer conductor 112 for tuning and impedance matching purposes. In the embodiment, as seen in FIGS. 1A and 1B, the design could be made to start with the inner conductor 114 concentrically centered relative to outer conductor 112 and then moved off center when the temperature changes. As shown in FIGS. 2A and 2B, inner conductor 114 may be normally off-center relative to outer conductor 112, and as the temperature increases, the inner conductor 114 moves toward the concentric center of the coaxial cable 110 when one of the materials 116, 118 is heated.

The system described in regard to FIGS. 1A-2B may include an electrosurgical generator 300 having a microprocessor and sensor circuitry (not shown) that continually monitors tissue impedance and measures offset impedance. The sensor circuitry may also continually monitor the position of the inner conductor 114 of a coaxial cable 110 with respect to a desired coaxial position (e.g., a center position). The monitor may be operably coupled to a mechanism (shape memory alloy, heat resistive element) as explained in more detail below) for regulating the thermal expansion of at least one of the first and second dielectric materials 116, 118 to position the inner conductor 114 relative to the outer conductor 112 to change the impedance of the inner conductor 114. The microprocessor or the circuitry may also be configured to compare the inner conductor positioning to a predetermined center position. If the inner conductor is positioned above or below the predetermined center position, one or more materials 116, 118 surrounding the inner conductor are heated or moved to re-position the inner conductor 114 to a desired position, and the microprocessor reports such findings to a user control or maintains this data for subsequent use.

FIG. 3 is a schematically illustrated cross-sectional view of a coaxial cable 210 having an inner conductor 214 held by one or more spacers 230, 232, 234 being composed of one or more materials having different coefficient of thermal expansion values. In FIG. 3, the coaxial cable 210 includes an outer conductor 212, an inner conductor 214, a first material 216, a second material 218, a first spacer 230, a second spacer 232 and a third spacer 234. The inner conductor 214 is connected to an external power source 300.

The first, second, and third spacers 230, 232, 234 maintain a desired position (e.g., a center position) for the inner conductor 214 for at least a partial length of the coaxial cable 210. Each of the spacers 230, 232, 234 may have the same or a different width, and each may be composed of one material or two or more materials. Also, the material used for each spacer may be different. For example, a first spacer 230 may be composed of a first material 216 and a second material 218, whereas the second and third spacers 232, 234 may be composed of one material.

FIG. 4A is a schematically illustrated cross-sectional view of an off-centered coaxial cable 310 and FIG. 4B is a schematically illustrated cross-sectional view of a centrally disposed coaxial cable 310 having an inner conductor and a plurality of resistive heating elements in each of two or more materials having different coefficient of thermal expansion values. In FIGS. 4A and 4B, the coaxial cable 310 includes an outer conductor 312, an inner conductor 314, a first material 316, a second material 318, first resistive heating elements 340 and second resistive heating elements 342.

FIG. 4A illustrates the inner conductor 314 in an off-centered position within the coaxial cable 310. As heat is applied via the heating resistive elements 340, 342 shown in FIG. 4B, the inner conductor 314 moves to a centered position due to the thermal expansion of material 318. As the tissue impedance changes, the alignment sensitivity of the cable 310 may be selectively changed (e.g., automatically or manually) such that the impedance of the cable 310 better matches the tissue impedance. One or more materials may be utilized to tune the inner conductor 314 according to a desired setting, such as an ohmage setting.

A plurality of first resistive heating elements 340 may be positioned in first material 316 and a plurality of second resistive heating elements 342 may be positioned in second material 318. The first and second resistive heating elements 340, 342 convert electricity into heat. Electrical current running through the elements encounter resistance, thus resulting in heating of the element. Resistive heating elements 340, 342 may be made from Nichrome which has a relatively high resistance and does not break down or oxidize in air at useful temperature ranges. First and second resistive heating elements 340, 342 may also be positioned in parallel to the inner conductor 314, at various lengths from the inner conductor 314, and in various widths. The temperature of each of the plurality of heating elements 340, 342 may be selectively controllable to position the inner conductor 314 relative to the outer conductor 312 and the plurality of heating elements 340, 342 may be disposed in a concentric array relative to the inner conductor 314.

FIG. 5A is a schematically illustrated cross-sectional view of an off-centered coaxial cable 410 and FIG. 5B is a schematically illustrated cross-sectional view of a centrally disposed coaxial cable 410. In FIGS. 5A and 5B, the coaxial cable 410 includes an outer conductor 412, an inner conductor 414, a dielectric material 416, and one or more resistive heating elements 440. In contrast to FIGS. 4A and 4B, only one dielectric material 416 is used to surround the entire length of the inner conductor 414. The dielectric material 416 includes one or more resistive heating elements 440 in parallel to the inner conductor 414 along the length of the cable 410. More particularly, the resistive heating elements 440 are positioned in parallel to the inner conductor 414, at various lengths along the inner conductor 414, and in various widths.

FIG. 5A illustrates the inner conductor 414 in an off-centered position within the dielectric material 416. As heat is applied, the inner conductor 414 is moved to a desired position (e.g., a center position) due to the thermal expansion of dielectric material 416 and due to first resistive heating element 440 b being heated to expand the dielectric material 416 in a given direction. Any member or combination of heating elements 440 a-440 e may be utilized to move the inner conductor 414 for tuning purposes. As the tissue impedance changes, the alignment sensitivity of the cable 410 may be selectively changed (e.g., automatically or manually) such that the impedance of the cable 410 better matches the tissue impedance.

FIG. 6 is a schematically illustrated cross-sectional view of a coaxial cable having an inner conductor with a shape memory alloy 550 in accordance with another embodiment of the present disclosure. In FIG. 6, the coaxial cable 510 includes an outer conductor 512, an inner conductor 514, a dielectric material 516 and a shape memory alloy 550.

The shape memory alloy 550 is, for example, positioned in proximity to the inner conductor 514. One or more shape memory alloys 550 may be positioned along the length of the coaxial cable 510 in predetermined distance from each other.

Shape memory alloys (SMAs) are a family of alloys having anthropomorphic qualities of memory and trainability and are particularly well suited for use with medical instruments. SMAs have been applied to such items as actuators for control systems, steerable catheters and clamps. One of the most common SMAs is Nitinol which can retain shape memories for two different physical configurations and changes shape as a function of temperature. Recently, other SMAs have been developed based on copper, zinc and aluminum and have similar shape memory retaining features.

SMAs undergo a crystalline phase transition upon applied temperature and/or stress variations. A particularly useful attribute of SMAs is that after it is deformed by temperature/stress, it can completely recover its original shape on being returned to the original temperature. The ability of an alloy to possess shape memory is a result of the fact that the alloy undergoes a reversible transformation from an austenitic state to a martenistic state with a change in temperature/stress. This transformation is referred to as a thermoelastic martenistic transformation.

Under normal conditions, the thermoelastic martenistic transformation occurs over a temperature range which varies with the composition of the alloy, itself, and the type of thermal-mechanical processing by which it was manufactured. In other words, the temperature at which a shape is “memorized” by an SMA is a function of the temperature at which the martensite and austenite crystals form in that particular alloy. For example, Nitinol alloys can be fabricated so that the shape memory effect will occur over a wide range of temperatures, e.g., −270° to +100° Celsius. Many SMAs are also known to display stress-induced martenisite (SIM) which occurs when the alloy is deformed from its original austensitic state to a martensitic state by subjecting the alloy to a stress condition.

As a result, when heat is applied to the coaxial cable 510, the inner conductor 514 tends to move from its desired position within the coaxial cable 510. SMA 550, which is embedded within a material 516 having a certain coefficient of thermal expansion and which is located in a close proximity to the inner conductor 514 may move the inner conductor 514 back to its desired position (e.g., a center position) within the coaxial cable 510. SMA 550 can recover from large amounts of bending and torsional deformations, due to the application of heat, as well as small amounts of strain. Provided the deformations are within recoverable ranges, the process of deformation and shape recovery can be repeated millions of times. As a result, the SMA 550 located within the material 516 can repeatedly move the inner conductor 514 back to a desired position (e.g., a centered position). Moreover, as can be appreciated, the material 516 may be designed to selectively (e.g., either automatically or manually) align or misalign the inner conductor 514 relative to the outer conductor 512 for tuning and impedance matching purposes.

Consequently, the embodiments of the present disclosure allow for improved antenna impedance matching for controlling tissue impedance of a microwave antenna during an ablation procedure via a thermally tuned coaxial cable. The embodiments further include changing the impedance of the coaxial cable for allowing greater flexibility in designing microwave antennas. By having a varying impedance of the coaxial cable in the antenna tuned to change with the increase/decrease in temperature, tissue impedance changes, and thus, the antenna may deposit a greater amount of energy over the entire course of the ablation procedure. By using dielectric cores of varying thermal expansion values, it is possible to force the eccentricity of the inner conductor of the coaxial cable on-line or off-line, thus effectively changing the coaxial cable's impedance value.

In addition, FIGS. 1A-2D illustrate two materials 16, 18 within the spacing between the inner surface of the outer conductor 12 and the outer surface of the inner conductor 14 including two air spaces or gaps 20, 22. However, one skilled in the art may use more than two materials within the spacing between the inner surface of the outer conductor 12 and the outer surface of the inner conductor 14 and more than two air gaps. For example, one skilled in the art may be motivated to use three or more materials, each with a different coefficient of thermal expansion value in a triangular configuration with three or more air gaps separating the materials. In addition, one skilled in the art may be motivated to use two materials in a checkered pattern or any other type of intertwined pattern with one or more air gaps in order to center or off-center the inner conductor 14 of the coaxial cable 10 as needed to tune or match the tissue impedance.

Further, in FIGS. 1A-6 there may be one or more mechanisms that regulate the thermal expansion of at least one of the first and second dielectric materials 16, 18 to position the inner conductor 14 relative to the outer conductor 12 to change the impedance of the inner conductor 14.

FIG. 7 shows another embodiment according to the present disclosure wherein the coaxial cable 700 includes an outer conductor 712 arranged to be generally concentric with respect to the inner conductor 714 used for transmitting signals. Inner conductor 714 is held relative to the outer conductor 712 by first material 716 and second material 718 only one of which contacts inner conductor 714. First material 716 and the second material 718 define first and second air gaps 720 and 722, respectively, between the inner surface of the outer conductor 712 and the outer surface of the inner conductor 714. A fluid 725 is circulated within one or both the first and second dielectric materials 716, 718 via conduits 727 and 729 defined respectively therein. The relative temperature of the fluid 725 may be selectively controllable via circuitry controlled by the generator 300 to regulate thermal expansion of one or both the first and second dielectric materials 716, 718 to position the inner conductor 714 relative to the outer conductor 712 to change the impedance of the inner conductor 714. The fluid 725 may optionally or alternatively be disposed between the first and second dielectric materials 716, 718 and be controlled in a similar manner.

While several embodiments of the disclosure have been shown in the drawings and/or discussed herein, it is not intended that the disclosure be limited thereto, as it is intended that the disclosure be as broad in scope as the art will allow and that the specification be read likewise. Therefore, the above description should not be construed as limiting, but merely as examples of particular embodiments. Those skilled in the art will envision other modifications within the scope and spirit of the claims appended hereto. 

1. A coaxial cable, comprising: an outer conductor and an inner conductor adapted to connect to an energy source; and first and second dielectric materials disposed between the inner conductor and the outer conductor that position the inner conductor relative to the outer conductor in a first position, wherein the first dielectric material has a first coefficient of thermal expansion and the second dielectric material has a second coefficient of thermal expansion different from the first coefficient of thermal expansion.
 2. The coaxial cable according to claim 1, wherein the first and second dielectric materials are thermally expandable and contractible to position the inner conductor relative to the outer conductor from the first position to a second position different from the first position.
 3. The coaxial cable according to claim 1, wherein the first and second dielectric materials are at least one of thermally expandable and contractible to position the inner conductor relative to the outer conductor from the first position wherein the inner conductor is concentrically aligned with the outer conductor to a second position wherein the inner conductor is not concentrically aligned with the outer conductor.
 4. The coaxial cable according to claim 1, wherein the first dielectric material is selected from the group consisting of ABS Polymer Extruded, ABS Polymer Nylon Blend, PEKK Polyketone, PEEK Polyketone, Nylon PTFE Filled, Polycarbonate Extruded, LDPE (Polyethylene), Polyimide, PTFE Molded, Silica Aerogel and combinations thereof.
 5. The coaxial cable according to claim 1, wherein the second dielectric material is selected from the group consisting of ABS Polymer Extruded, ABS Polymer Nylon Blend, PEKK Polyketone, PEEK Polyketone, Nylon PTFE Filled, Polycarbonate Extruded, LDPE (Polyethylene), Polyimide, PTFE Molded, Silica Aerogel and combinations thereof.
 6. The coaxial cable according to claim 1, wherein the first and second dielectric materials define at least one space disposed therebetween.
 7. The coaxial cable according to claim 1, further comprising at least one of a shape memory alloy and a resistive heating element operatively associated with at least one of the first and second dielectric materials for regulating the thermal expansion of at least one of the first and second dielectric materials to position the inner conductor relative to the outer conductor.
 8. The coaxial cable according to claim 1, wherein a fluid is disposed between the first and second dielectric materials, the relative temperature of the fluid being selectively controllable to regulate thermal expansion of at least one of the first and second dielectric materials to position the inner conductor relative to the outer conductor.
 9. The coaxial cable according to claim 1, wherein at least one heating element is disposed within at a least one of the first and second dielectric materials, the relative temperature of the heating element being selectively controllable to regulate thermal expansion of at least one of the first and second dielectric materials to position the inner conductor relative to the outer conductor.
 10. The coaxial cable according to claim 1, wherein the first dielectric material holds the inner conductor and the second dielectric material mechanically supports the first material.
 11. The coaxial cable according to claim 1, wherein the energy source includes sensor circuitry that monitors at least one of the inner and outer conductors for determining the position of the inner conductor relative to the outer conductor.
 12. The coaxial cable according to claim 11, wherein the sensor circuitry is operably coupled to a means for regulating the thermal expansion of at least one of the first and second dielectric materials to position the inner conductor relative to the outer conductor to change the impedance of the inner conductor, the means for regulating including at least one of a shape memory alloy, a heat resistive element and a temperature controllable fluid.
 13. A coaxial cable, comprising: an outer conductor and an inner conductor adapted to connect to an energy source; and at least one dielectric spacer disposed between the outer conductor and an inner conductor, the dielectric spacer including first and second dielectric materials disposed between the inner conductor and the outer conductor that position the inner conductor relative to the outer conductor in a first position, wherein the first dielectric material has a first coefficient of thermal expansion and the second dielectric material has a second coefficient of thermal expansion different from the first coefficient of thermal expansion.
 14. The coaxial cable according to claim 13 further comprising a plurality of dielectric spacers spaced relative to one another along a length of the cable.
 15. A coaxial cable, comprising: an outer conductor and an inner conductor adapted to connect to an energy source; at least one dielectric material disposed between the outer conductor and an inner conductor, wherein the dielectric material is thermally expandable to position the inner conductor relative to the outer conductor from a first position wherein the inner conductor is not concentrically aligned with the outer conductor to a second position wherein the inner conductor is more concentrically-aligned with the outer conductor; and at least one heating element disposed within the dielectric material adapted to connect to an energy source for controlling the temperature thereof.
 16. A coaxial cable according to claim 15 further comprising a plurality of heating elements disposed within the dielectric material, the temperature of each of the plurality of heating elements being selectively controllable to position the inner conductor relative to the outer conductor.
 17. A coaxial cable according to claim 16 wherein the plurality of heating elements is disposed in a concentric array relative to the inner conductor.
 18. A coaxial cable according to claim 15, wherein the generator further comprises sensor circuitry which monitors the position of the inner conductor within the coaxial cable.
 19. A method for controlling the impedance of a coaxial cable, the method comprising: providing: an outer conductor and an inner conductor; first and second dielectric materials disposed between the inner conductor and the outer conductor, wherein the first dielectric material has a first coefficient of thermal expansion and the second dielectric material has a second coefficient of thermal expansion different from the first coefficient of thermal expansion; energizing the cable; determining an impedance of the inner conductor; and causing at least one of the first and second dielectric materials to thermally expand or contract to move the inner conductor relative to the outer conductor to change the impedance of the coaxial cable.
 20. A method according to claim 19 wherein the impedance of the coaxial cable is substantially matched to the impedance of at least one of a microwave generator and a microwave probe. 